Both proper and common nouns are defined as a person, place, or thing. The difference comes in the fact that common nouns can be any person, any place, or any thing. A girl, a mountain, or a can. These words tell what something is but aren't specific.
A proper noun, on the other hand, is a particular noun, usually a name of sorts. Instead of 'a girl,' the proper noun could be 'Sarah' Likewise, 'a mountain' could be Pike's Peak. Proper nouns can be the name of any person, place, thing, or organization. The first letter of a proper noun is always capitalized.